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Ind. Eng. Chem. Res. 2008, 47, 4754–4757
Activated Carbons Obtained from Rice Husk: Influence of Leaching on Textural Parameters Cristina Deiana,† Dolly Granados,† Rosa Venturini,† Alejandro Amaya,‡ Marta Sergio,‡ and Ne´stor Tancredi*,‡ Instituto de Ingenierı´a Quı´mica, Facultad de Ingenierı´a, UniVersidad Nacional de San Juan, AV. Libertador 1109 oeste, CC 5400, San Juan, Argentina, and Laboratorio de Fisicoquı´mica de Superficies y Ca´tedra de Fisicoquı´mica, DETEMA, Facultad de Quı´mica, UniVersidad de la Repu´blica Oriental del Uruguay, General Flores 2124, CC 1157, MonteVideo, Uruguay
Activated carbons obtained from rice husk exhibit low specific surface areas when physical activation is applied due to its high silica content. The purpose of this work was to improve textural parameters of powdered activated carbons obtained from rice husk. To avoid the negative influence of the raw material ash content, a leaching step was included in the preparation process. Hydrofluoric acid, in two concentrations (25 and 50 wt %), was used as leaching agent and applied in different stages of the process. Physical activation using water vapor as activating agent was applied. Specific surface area and porosity were evaluated from nitrogen adsorption data. When a leaching step was included, specific surface area values between 700 and 1200 m2/g were obtained. These values are higher than that corresponding to the activated carbon prepared from rice husk not treated with acid (290 m2/g). Pore size distribution depends on the step sequence of the preparation process and on the HF concentration. Introduction Almost any carbonaceous material, with high carbon content and low proportion of inorganic components, can be used as precursors for the preparation of activated carbons.1 Many agricultural solid wastes have traditionally been used as raw materials for this purpose, among them coconut shell,2 olive stones,3 eucalyptus wood,4 and corn cob.5 The increase of agricultural activity produces large volumes of solid wastes that, in many cases, become potential environmental contaminants. Rice constitutes the base of diet for a great part of humanity. Rice husk waste represents 20% of rice production and grows up to many tons per year. Even though it has been used for land filling and fuel or biogas production, new technical applications have been explored in the last years.6,7 The major constituents of rice husk are cellulose, lignin, and mineral components. The content of each of them depends on rice variety, climatic conditions, and even the geographic localization of the culture. When rice husk is subjected to combustion at moderate temperature, an amorphous ash is obtained; a 95% white silica powder could be produced after calcination at 700 °C for 6 h.6 Many authors have concluded that rice husk is an excellent source of amorphous silica of high purity appropriate to produce silicium, silicium nitride, silicium carbide, or magnesium siliciure.8 As a consequence it constitutes a non-traditional raw material for the ceramic industry. As a result of its high cellulose and lignin content, rice husk can be used as the raw material to prepare activated carbons, which are known to be highly complex porous structures, with high values of specific surface area (ssa) and porous sizes mainly in the range of micropores. They are obtained by two different processes: the “physical” or “thermal” activation and the “chemical” activation. In the former carbonization is followed by char activation; in the second one, carbonization and * Author to whom correspondence should be addressed. E-mail:
[email protected]. Tel: +598 2 9248352. Fax: +598 2 9241906. † Universidad Nacional de San Juan. ‡ Universidad de la Repu´blica Oriental del Uruguay.
activation are performed in a single step, using a chemical agent. Activated carbons are very effective adsorbents due to their textural parameters: ssa and microporous structure. Physical activation of rice husk produces activated carbon that exhibits very low ssa’s.9 This fact has been principally attributed to the high ash content of the raw material. Rice culture in Uruguay produces up to 160 to 200 thousands of tons of rice husk per year. Nowadays, only a part of it finds a profitable final disposal; used in the best cases as an energetic source in different industrial processes or as chicken or horse beds. Otherwise, and up to the half of the total production, it is disposed as a solid residue in pourer spaces when not burned in open sky. The purpose of this work is to improve textural parameters of powdered activated carbons obtained from rice husk. To avoid the negative influence of the raw material ash content, leaching steps are included in the preparation process, either before or after the activation step. As ashes are mostly constituted by silica, hydrofluoric acid is used as the leaching agent, whose efficiency to dissolve silica is well-known.10 Textural parameters, ssa, and porosity are evaluated from nitrogen adsorption data. Materials and Methods Rice husk provided by the Uruguayan firm Saman was used as raw material. Elemental analysis was performed using Carlo Erba equipment, EA 1108 CHNS-O model. C, H, N, S, and O contents refer to dry mass. Oxygen content was determined by difference and ash content by calcination of the sample at 900 °C during 24 h. Carbonization. A sample of 1500 g of rice husk was calcined in a batch process using a retort-like stainless steel reactor, under inert atmosphere of nitrogen, at a rate of 1.4 K/min from room temperature up to 773 K and keeping the final temperature for 2 h.11 A K-type thermocouple and a digital temperature controller were used to set and control the sample temperature. The resulting solid was labeled as C, and the yield refers to the mass of the starting dry raw material.
10.1021/ie071657x CCC: $40.75 2008 American Chemical Society Published on Web 06/13/2008
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4755 Scheme 1. Experimental Scheme and Sample Identification
Table 1. Ultimate Analysis for Rice Husk and Derived Products (%, Dry Basis, Ash Free) and Ash Content (%, Dry Basis) sample
C
N
H
S
Oa
ash
rice husk C CA CAL50 CAL25 CL50A CL25A CL50 CL25 L50CA
48.3 85.7 83.1 95.0 83.7 75.7 86.0 89.1 83.5 90.0
6.8 1.0 0.5 0.0 0.4 0.6 0.5 0.9 1.1 0.6
0.5 3.1 1.9 4.2 1.5 1.2 1.2 3.0 2.9 1.6
0 0 0 0 0 0 0 0 0 0
44.4 10.2 14.5 0.8 14.4 22.5 12.3 7.0 12.5 7.8
17.2 43.2 68.4 1.0 0.0 1.0 1.1 0.0 0.8 0.0
a
By difference.
Table 2. Textural Parameters and Yields for Activated Carbons Obtained from Rice Husk textural parameters
Activation. The sample in the form of powder was heated in a 300 mm long stainless steel reactor with an internal diameter of 30 mm, using an electric oven. The temperature was set and controlled using a K-type thermocouple and a digital temperature controller.12 The sample was heated at 5 K/min from room temperature to 1123 K in nitrogen gas flowing at 300 mL/min. When that temperature was reached, nitrogen was substituted for water vapor as activating agent at a flowing rate of 1.7 g per gram of carbonized matter per hour. This treatment last for 105 min. Samples subjected to this process were labeled as A. Leaching. Hydrofluoric acid of analytic grade was used at room temperature. The acid solution volume to solid mass ratio was 2.5:1, and two different concentrations of acid (25% and 50%, w/w) were used. The slurry was magnetically stirred for 30 min. The solid was separated and washed with distilled water until neutralization, and then oven-dried at 373 K during 16 h. The corresponding samples were labeled as LX, where X (25 or 50) represents acid concentration. Leaching treatment was performed before or after the activation one. The carbonized rice husk was subjected to activation and leaching following the experimental diagram shown in Scheme 1. Sample identifications are also shown in the scheme. For the sake of comparison an additional sample was obtained by leaching the rice husk with 50% HF and then carbonizing and activating the solid as described previously. This sample was labeled L50CA. Characterization. Nitrogen adsorption-desorption isotherms were performed using Autosorb-1 (Quantachrome) equipment. Samples of about 0.100 g were oven-dried at 378 K during 24 h and outgassed at room temperature, under vacuum, during other 24 h. The final pressure was less than 10-4 mbar. Textural parameters were derived from adsorption data. The ssa was determined based on the BET model.13 The specific total pore volume (VT) was determined from the adsorption at the relative pressure of 0.99, converted to liquid volume assuming a nitrogen density of 0.808 g/cm3. The specific micropore volume (VDR) was determined using the Dubinin-Radushkevich model.14 The pore size distribution (PSD) was analyzed using the DFT method.15,16 The total yield (YT) was calculated as YT ) (obtained weight of the sample ⁄ weight of the untreated rice husk) × 100 Results and Discussion Results of rice husk and the derived products ultimate analysis are listed in Table 1. Textural parameters determined after adsorption data are shown in Table 2.
sample
YT (wt %)
ssa BET (m2/g)
VT (cm3/g)
VDR (cm3/g)
C CL25 CL50 CA CL25A CL50A CAL25 CAL50 L50CA
41.0 23.4 23.2 24.3 14.1 12.1 6.9 6.9 11.8
16 170 160 290 750 820 1030 1180 950
0.07 0.18 0.17 0.25 0.54 0.60 0.98 1.09 0.46
0.06 0.05 0.12 0.31 0.35 0.39 0.50 0.39
Ultimate analysis showed that for most of the processes the chemical composition of the products did not change significantly. However, when lixiviation with 50% HF is involved, some results are remarkable. The lixiviation with 50% HF seems to increase the carbon percentage and to decrease the oxygen percentage (see the series C, CL50 and CA, CAL50) perhaps through the removal of surface oxygenated functions. On the contrary, the activation of CL50 seems to increase the oxygen content, and consequently a decrease in the carbon percentage is observed (compare CL50A and CL50), probably as a result of the inclusion of oxygenated groups through water vapor activation. Carbonization of the sample gave a solid (C) that presented very low values of VT (0.07 cm3/g) and ssa (16 m2/g). These results suggest that carbonization does not generate any porous structure. Leaching of the char C using hydrofluoric acid produced solids (CLx) with higher ssa and VT; they were respectively 10fold and twice those of the char. At this stage, the concentration of the acid did not cause any difference in these textural parameters of the solids. Activation of the char (CA) increased the ssa (18 times) and the VT (3.5 times) and generated a VDR that represents half of VT. It is well-known that physical activation generates the porous network of activated carbon, widening the primary pores, opening the secondary ones, and contributing to higher values of textural parameters.1 Activation of leached solids (CLxA samples) had a more drastic effect: the increase of the ssa was 4.4 times for CL25A and 5.1 times for CL50A, referenced to the parent CLx sample. The VT increase was 3 and 3.5 times and the VDR one was 5 and 7 times that of the CLx samples, using respectively, 25% and 50% HF. In this case, when leaching was performed on the char and before activation, some influence of the concentration of the acid was observed. The higher concentration produced solids with ssa and VDR that are 2.8 times those of CA. The results suggest that when the leaching step is performed before the activation one, inorganic salts are removed, probably
4756 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
Figure 1. Evolution of the ssa as a function of YT; sample C is not included. Figure 3. PSD for samples CL25A and CL50A, DFT theory.
Figure 2. PSD for samples CL25 and CL50, DFT theory. Figure 4. PSD for samples CAL25, CAL50, and CA, DFT theory.
allowing the bulk penetration of water vapor and generating a more developed pore network. Leaching of the activated samples (CALx samples) produced solids that presented the highest values of textural parameters. The leaching process increased the ssa of the activated samples 3.5 or 4 times, for concentration of the acid of 25% or 50% respectively, referenced to CA. The VDR increased in the same ratio. Possibly the porous network produced by the activation process allows the hydrofluoric acid to penetrate deeply in the sample, dissolving silica. The leaching step (50% HF) performed before the carbonization-activation process to compare the results with those previously described gave a solid (L50CA) with ssa and VDR values that are, respectively, 16 and 11% higher than those of CL50A. These results are discussed later. From the point of view of the development of the ssa and the VDR, the process sequence carbonization-activation-leaching is more efficient than the sequence carbonization-leachingactivation. Figure 1 shows the variation of the ssa as a function of YT: a lineal correlation is observed between them. In fact, the leaching process, as well as the activation one, generates the porous structure by mass subtraction of the sample that is lost in solution or as gas products, respectively. Figures 2–5 show the PSD diagrams for the studied samples. Only pore sizes over 10 Å are analyzed taking into account literature reports on the artificial gap in the vicinity of 10 Å. 17 In all the cases, there are no peaks for pore sizes over 30 Å. For samples CL25 and CL50 the PSDs are very similar (peaks at 18 Å and 12 Å in Figure 2) according with the close values for ssa, VT and VDR for both samples. The activation of these samples increases pore volumes; the CL25A shows only a high and wide peak at about 14 Å and a small one at 18 Å, and the
Figure 5. PSD for samples CA and L50CA, DFT theory.
CL50A shows two important peaks at 12 Å and 16 Å (Figure 3). The activation process of the CL25 widened the 12 Å pores to 14 Å but seems not to have altered the 18 Å pores. For CL50 the activation process increased the 12 Å pore size volume and generated 16 Å pores. The leaching process with higher acid concentration removed mineral matter from the char in a higher extension allowing the activation process to go deeply and to widen the pores. PSD for CA shows a peak at 16 Å with a shoulder at 13 Å (Figure 4). Leaching of this sample with HF splits this peak in three: two small ones at 18 Å and 23 Å and a high and wide one at 14 Å. Leaching seems to open pore mouths, from 13 Å to 14 Å and from 16 Å to 18 Å and 23 Å. The acid concentration
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4757
of the leaching step when performed after activation does not seem to influence the PSD profile of the samples. Leaching of rice husk previous to carbonization and activation steps allows the formation of pores of 13 Å and 16 Å (Figure 5). These two values are coincident with the major peak and the shoulder present in the PSD of the CA sample. The PSD for L50CA (Figure 5) is coincident with that for CL50A (Figure 3), suggesting that when 50% HF is used, the sequence leaching-carbonization-activation gives results similar to those of the carbonization-leaching-activation one. The higher peak areas for L50CA than for CL50A are a result of the higher VDR of the former solid, as was already indicated. Conclusions Rice husk, a residue of the agricultural activity representing high volumes of a potential contaminant, can be transformed in powdered activated carbons with high ssa and developed microporous structure. This is accomplished by the introduction of a leaching step in the preparation process. The process sequence carbonization-activation-leaching is more efficient than the sequence carbonization-leaching-activation to improve the ssa and the VDR. Previous leaching of the rice husk with 50% HF followed by carbonization and activation gives solids with textural parameters similar to those obtained by the sequence carbonization-activation-leaching. PSD is affected in different extensions, not only by the step sequence of the preparation process but also by the HF concentration. Acknowledgment The authors are grateful to Departamento Estrella Campos, Facultad de Quı´mica, Universidad de la Repu´blica, Uruguay, for their assistance with ultimate chemical analysis, and to Universidad Nacional de San Juan, Argentina, for financial support. Literature Cited (1) Rodrı´guez-Reinoso, F. Carbons. In Handbook of Porous Solids; Schu¨th, F., Sing, K., Weitkamp, J. Eds.; Wiley-VCH: Weinheim, Germany, 2002; Vol. 3, pp 1766-1827.
(2) Pandolfo, A. G.; Amini-Amoli, M.; Killingley, J. S. Activated Carbons prepared from Shells of different Coconut Varieties. Carbon 1994, 32, 1015–9. (3) Rodrı´guez-Valero, M. A.; Martı´nez-Escandell, M.; Molina-Sabio, M.; Rodrı´guez-Reinoso, F. CO2 Activation of Olive Stones Carbonized under Pressure. Carbon 2001, 39, 287–324. (4) Tancredi, N.; Cordero, T.; Rodrı´guez-Mirasol, J.; Rodrı´guez, J. Activated Carbon from Uruguayan Eucalyptus Wood. Fuel 1996, 75, 1701–6. (5) Chang, C. F.; Chang, C. Y.; Tsai, W. T. Effects of Burn-Off and Activation Temperature on Preparation of Activated Carbons from Corn Cob Agrowaste by CO2 and Steam. J. Colloid Interface Sci. 2000, 232, 45–9. (6) Della, V. P.; Ku¨hn, I.; Hotza, D. Rice Husk Ash as an Alternate Source for Active Silica Production. Mater. Lett. 2002, 57, 818–21. (7) Park, B.; Wi, S. G.; Lee, K. H.; Singh, A. P.; Yoon, T.; Kim, Y. S. Characterization of Anatomical Features and Silica Distribution in Rice Husk using Microscopic and Micro-Analytical Techniques. Biomass Bioenergy 2003, 25, 319–27. (8) Yalcin, N.; Sevinc, V. Studies on Silica obtained from Rice Husk. Ceram. Int. 2001, 27, 219–24. (9) Yalcin, N.; Sevinc, V. Studies of the Surface Area and Porosity of Activated Carbons prepared from Rice Husk. Carbon 2000, 38, 1943–45. (10) Burriel Martı´, F.; Lucena, C. F.; Arribas Jimeno, S.; Herna´ndez Mendez, J. Quı´mica Analı´tica CualitatiVa; Paraninfo Thomson Editores: Madrid, Spain, 2001. (11) Deiana, A. C.; Granados, D. L.; Petkovic, L. M.; Sardella, M. F.; Silva, H. Use of Grape Must as Binder to Obtain Activated Carbon Briquettes. Braz. J. Chem. Eng. 2004, 21, 585–91. (12) Deiana, A. C.; Noriega, S. E.; Petkovic, L. M. Carbo´n Activado a Partir de Materias Primas Regionales. Inf. Tecnol. 1998, 9, 89–94. (13) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309–19. (14) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders & Porous Solids. Principles, Methodology and Applications; Academic Press: London, U.K., 1999. (15) Seaton, N. A.; Walton, J.P.R.B.; Quirke, N. A New Analysis Method for the Determination of the Pore Size Distribution of Porous Carbons from Nitrogen Adsorption Measurements. Carbon 1989, 27, 853– 61. (16) Ryu, Z.; Zheng, J.; Wang, M.; Zhang, B. Characterization of Pore Size Distributions on Carbonaceous Adsorbents by DFT. Carbon 1999, 37, 1257–64. (17) Ustinov, E. A.; Do, D. D. Application of Density Functional Theory to Analysis of Energetic Heterogeneity and Pore Size Distribution of Activated Carbons. Langmuir 2004, 20, 3791–7.
ReceiVed for reView December 4, 2007 ReVised manuscript receiVed April 17, 2008 Accepted April 30, 2008 IE071657X
